1. Field of the Invention
The present invention relates to a resonant circuit, a filter circuit, and an antenna device.
2. Related Art
A communication apparatus which performs information communication by radio or wire is constituted by various high frequency components such as an antenna, an amplifier, a mixer, and a filter. Among these components, a band pass filter (BPF), in which a plurality of resonant elements are arranged, has a function of passing only a signal in a specific frequency band. In today's communication systems, from a viewpoint of effective use of frequency, a sharp cut-off characteristic is preferred as a filter characteristic so as to enable the maximum use of the available band width. Further, to meet a demand for miniaturization of communication apparatuses, a filter having a smaller size is preferred.
In order to realize the filter characteristics, it is necessary to make a plurality of resonant elements coupled by electromagnetic fields, and the circuit constant of the filter consists of the resonant frequency fi of each resonant element, the coupling coefficient between resonant elements Mij, and the external quality factor Qe.
Methods for realizing the coupling coefficient between resonant elements in a filter circuit can be roughly classified into the following two kinds. The first method is a gap coupling by which a desired coupling is realized only on the basis of the positional relation between resonant elements without adding a coupling element in addition to the resonant elements. The gap coupling is suitable for a filter circuit which is constituted only by the coupling between adjacent resonant elements, such as in the Chebyshev's function type filter. The second method is a cross coupling by which a coupling is realized by adding a transmission line as described in the Patent Document 1 and the Patent Document 2. The cross coupling is suitable for a filter circuit which makes the steep skirt characteristics by the attenuation pole, and improving the planarity in group delay.
In a planar filter in which all resonant elements are arranged on a same plane, it is difficult to take a sufficient interval between adjacent resonant elements when promoting the miniaturization of the filter, as a result of which undesirable cross couplings exist in addition to desired couplings. By the influence of the undesirable cross couplings, the filter performance or the symmetry of the filter cut-off characteristic is deteriorated, which is one of the causes of the difficulty in realizing the filter characteristics.
As a measure against the undesirable cross coupling, there are a method for making the magnitude of the undesirable cross coupling small by devising the shape and arrangement of the resonant elements, and a method for effecting electromagnetic shielding between resonant elements which are coupled with each other by an undesirable cross coupling, by inserting a metal plate or the like between the resonant elements, as described in the Patent Document 3 and the Patent Document 4.
Patent Document 1: JP-A 2004-530391 (Kokai)
Patent Document 2: JP-A 2000-341071 (Kokai)
Patent Document 3: JP-A 2001-308603 (Kokai)
Patent Document 4: JP-A 2004-349966 (Kokai)
As described above, the undesirable cross coupling is not controlled by the prior art, and in the case where there are structural restrictions, such as those in miniaturizing a filter and an antenna, there are problems that the filter characteristic, the voltage standing wave ratio (VSWR), and the gain of the antenna are deteriorated by the undesirable cross coupling.
An object of the present invention is to provide a resonant circuit, a filter circuit, and an antenna device, in which the above described performance is improved by eliminating the above described disadvantages of the prior art, and by controlling the uncontrolled cross coupling between resonant elements which constitute the filter and the antenna.